385005332-LTE-Mastering-pdf.pdf

Mastering LTE
Air Interface
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Table of Contents
LTE_301 Version 1.9 i
Chapter 1
Introduction to LTE..........................................................................................................................................1
Goals and Requirements................................................................................................................................. 3
Network Architectures and Interfaces............................................................................................................ 7
Air Interface....................................................................................................................................................11
LTE Protocols and Interfaces ........................................................................................................................14
LTE Devices....................................................................................................................................................17
Chapter 2
LTE Air Interface Essentials..........................................................................................................................23
OFDMA and SC-FDMA....................................................................................................................................25
LTE Frame Structure......................................................................................................................................30
LTE Channels and Signals.............................................................................................................................37
Multiple-Antenna Techniques .......................................................................................................................45
Chapter 3
System Acquisition........................................................................................................................................55
Overview of System Acquisition....................................................................................................................57
Processing of Synchronization Signals and PBCH.......................................................................................61
Acquiring SIBs ................................................................................................................................................69
Additional Material.........................................................................................................................................80

Table of Contents
ii
Chapter 4
System Access...............................................................................................................................................85
Random Access Procedure Overview...........................................................................................................87
Random Access Parameters.........................................................................................................................91
RRC Connection Establishment..................................................................................................................102
Chapter 5
Data Session Setup.....................................................................................................................................107
Overview of Initial Attach.............................................................................................................................109
Default EPS Bearer Setup...........................................................................................................................116
Chapter 6
Downlink Operations...................................................................................................................................125
Channel Quality............................................................................................................................................128
Hybrid ARQ ...................................................................................................................................................146
Additional Material.......................................................................................................................................151
Chapter 7
Uplink Operations........................................................................................................................................163
Overview of UL Transmission......................................................................................................................165
Request for UL Resources...........................................................................................................................169
UL Grant Allocation......................................................................................................................................173
BSR Reporting..............................................................................................................................................175
UL Grant for Data Transmission .................................................................................................................178
Transmission on the PUSCH .......................................................................................................................180

Table of Contents
iii
UL HARQ .......................................................................................................................................................187
Additional Material.......................................................................................................................................193
Chapter 8
Mobility and Power Control ........................................................................................................................201
Mobility .........................................................................................................................................................203
Cell Selection and Reselection ...................................................................................................................208
Tracking Area Update ..................................................................................................................................213
Handover......................................................................................................................................................216
Power Control...............................................................................................................................................228
Appendix A
OFDM Essentials .........................................................................................................................................237
Acronyms .....................................................................................................................................................251
References...................................................................................................................................................257

1 | Introduction to LTE
Award Solutions Proprietary
Chapter 1:
Introduction to LTE
1

References:
[1] 3GPP TS 36.300 – E-UTRA and E-UTRAN Overall
Description (Stage 2)
[2] 3GPP TS 36.211 through 36.214: Physical Layer
related documents
Objectives
After completing this module, you will be able to:
• Discuss the goals and requirements of LTE
• Sketch the LTE access and core network
architectures and interfaces
• Describe the key functions of the LTE air
interface
• Explain the steps of a typical LTE call
2

Goals and
Requirements
3

The 3G Partnership Project (3GPP) is responsible for
defining the Long Term Evolution program for 3GPP
networks, called LTE. 3GPP focuses on three key areas:
• Evolved Universal Terrestrial Radio Access (E-UTRA):
This air interface is based on an OFDM physical layer
and utilizes MIMO techniques to increase the data
rates. It supports more than 300 Mbps in the
downlink to the User Equipment (UE) and more than
50 Mbps in the uplink, using a scalable channel
bandwidth of up to 20 MHz.
• Evolved Universal Terrestrial Radio Access Network
(E-UTRAN): Unlike the Node B and Radio Network
Controller (RNC) of the UTRAN, the E-UTRAN has only
one node: the evolved Node B, or eNB. The eNB is
responsible for the physical layer operations of OFDM
and MIMO, and is also responsible for scheduling of
downlink and uplink resources, handovers, and Radio
Resource Management (RRM).
• Evolved Packet Core (EPC): 3GPP R99 through R6
used circuit (Mobile Switching Center, MSC) and
packet (Serving GPRS Support Node, SGSN, and
Gateway GPRS Support Node, GGSN) core network
components. In LTE, the network is moving to
simplified IP-based networks, replacing the current
network components with Mobility Management
Entities (MMEs) and Serving Gateways (S-GWs) and
Packet Data Network Gateways (P-GWs).
EPC
LTE: Long Term Evolution
E-UTRAN
UE
eNB
eNB
E-UTRA
• Downlink: 300 Mbps
• Uplink: 75 Mbps
• OFDM and MIMO
E-UTRAN
• Simplified architecture
• Evolved Node B
Evolved Packet Core (EPC)
• Simplified architecture
• IP-based services
MME/S-GW P-GW
E-UTRAN + EPC= EPS (Evolved Packet System)
4

For LTE, the evolution process has been a while in the
making, and is not likely to end anytime soon. Each 3GPP
standards release since the original UMTS specification
has continued to add to and expand the capabilities of the
network:
• Release 99 (R99) defined the original UMTS system,
supporting circuit voice services as well as theoretical
peak data rates of up to 2 Mbps. (Commercial
systems delivered packet data services of up to 384
kbps.)
• R4 defined a bearer-independent circuit-switched
architecture, separating switches into gateways and
controllers, and laying the groundwork for the IP
Multimedia Subsystem (IMS).
• R5 defined High Speed Downlink Packet Access
(HSDPA), which boosted packet data rates to 14
Mbps on the downlink. R5 also completed the design
of IMS.
• R6 increased data rates to more than 5 Mbps on the
uplink with High Speed Uplink Packet Access
(HSUPA), and introduced support for multimedia
broadcast/multicast services (MBMS).
• R7 provided further enhancements to HSDPA and
HSUPA, called HSPA+. Support for higher-order
modulation and Multiple Input Multiple Output
(MIMO)-antenna systems offered a significant
increase in data rates, potentially up to 42 Mbps.
• R8 defined the Long Term Evolution (LTE) system,
starting the transition to 4G technology, while R9
adds further enhancements and capabilities,
including support for MBMS, the definition of Home
eNBs for improved residential and in-building
coverage, and support for IMS-based emergency
calls.
Even as operators are rolling out the LTE systems, 3GPP is
working on additional improvements to LTE. In particular,
R10 introduces LTE-Advanced, offering support for (8x8)
MIMO in the downlink, channel aggregation up to 100
MHz, and relays. LTE-Advanced would continue to evolve
in Release 11 and beyond.
R 99
R 4
R 5
R 6
R 7
R 8
R 9
R 10
Release 4
Bearer-independent
CS architecture
Release 99
Voice, 2 Mbps (384
kbps) data rate
Release 5
HSDPA (14 Mbps DL)
Release 6
HSUPA (5.76 Mbps UL)
Release 7
HSPA+ (21/28 Mbps)
Release 8
LTE (300 and
75 Mbps)
Release 9
Emergency calls
using IMS
Release 10
LTE-Advanced
(1 Gbps and 500 Mbps)
3GPP Evolution
5

The Evolved UTRAN (E-UTRAN) is designed to meet a
number of very challenging performance goals, in order to
meet the evolving expectations of the subscribers and the
operators.
• Scalability: The system must be deployable in
markets with different available bandwidths, ranging
from 1.4 MHz to 20 MHz.
• Latency: Latency in the Control Plane (C-Plane) for
idle-mode to active-mode transitions must be less
than 100 ms, while the User Plane (U-Plane) delay
must be less than 10 ms (Ex: one-way eNB-to-UE
delay). A simple 3GPP analysis shows that a U-plane
delay of even 5 ms is achievable.
• Data Rates: Peak data rates must be at least 300
Mbps on the downlink and 50 Mbps on the uplink. On
average, user data rates should be three to four times
what HSDPA offers, and two to three times what
HSUPA can provide.
• Inter-RAT Handover Delays: Handover of real-time
services must take less than 300 ms, while non-real-
time applications must take less than 500 ms.
• Cell Coverage: Performance targets must be met out
to a cell radius of 5 km. Beyond that, no more than a
slight degradation is allowed out to 30 km.
• Mobility: The system must be optimized for relatively
low speeds (< 15 km/h), but should be able to
maintain active connections up to 500 km/h.
Basically, advanced antenna techniques and link
adaptation techniques work well at lower speeds.
E-UTRAN Performance Goals
Scalable Bandwidth
• 1.4/3/5/10/15/20 MHz
Latency
• < 100 ms (C-Plane)
• < 10 ms (U-Plane)
Inter-RAT Handover Delays
• < 300 ms (real-time)
• < 500 ms (non-real-time)
Mobility
• Optimized for low speeds (< 15 km/h)
• Connections maintained at high speeds
(up to 500 km/h)
Coverage
• Meet performance targets up
to 5 km
• Slight degradation up to 30 km
Data Rates
• 300 Mbps (DL) and 75
Mbps (UL) peak
• Three to four times
HSDPA and two to three
times HSUPA on average
6

Network Architectures
and Interfaces
7

The primary difference between the UTRAN and E-UTRAN
architectures is the absence of a Radio Network Controller
(RNC). The functionality of the RNC has now been moved
into the eNBs.
An eNB is responsible for the following functions:
• Radio Resource Management (RRM) functionalities
like radio bearer control and radio admission control,
• IP header compression and encryption of the user
data stream,
• Uplink/downlink radio resource allocation,
• Transfer of paging messages over the air,
• Transfer of Broadcast Control Channel (BCCH)
information over the air,
• Selection of the Mobility Management Entity (MME)
during a call,
• Mobility control in the active state,
The eNBs are connected to the MME and Serving
Gateways (MME/S-GW) via the S1 interface. An eNB is
able to communicate with multiple gateways, in order to
enable load sharing and redundancy. eNBs are
interconnected by the X2 interface, to coordinate
handovers and data transfers.
eNB and E-UTRAN
UE
eNB
eNB
MME
S-GW
X2
S1-MME
eNB
• Radio resource
management
• Header compression
• Encryption
• BCCH information
• Paging
• Mobility in active state
• MME selection
E-UTRAN
• No centralized controller
(RNC)
• eNBs linked via X2
interface
E-UTRAN
Uu
S1-U
8

New entities in the Evolved Packet Core (EPC) include the
Mobility Management Entity (MME), the Serving Gateway
(S-GW), and the Packet Data Network (PDN) Gateway (P-
GW).
• MME: The MME is responsible for managing and
storing UE contexts, generating temporary identifiers
to the UEs, idle-state mobility control, distributing
paging messages to eNBs, security control, and
Evolved Packet System (EPS) bearer control.
• Gateways: There are two gateways in LTE, one facing
toward the E-UTRAN (the S-GW) and one facing
toward the external packet data network (the P-GW).
A UE has only one S-GW but it may have multiple P-
GWs.
• Serving Gateway: The S-GW is responsible for
anchoring the user plane for inter-eNB handover and
inter-3GPP mobility, similar to a SGSN in a pre-LTE
UMTS network.
• PDN Gateway: This gateway is responsible for
anchoring the user plane for mobility between 3GPP
access systems and non-3GPP access systems.
Similar in nature to a Home Agent in Mobile IP or
GGSN in a pre-LTE UMTS network, the P-GW allocates
the user’s IP address, and forwards packets intended
for the user to the appropriate S-GW. It also provides
support for charging, lawful interception and policy
enforcement.
The EPC connects to auxiliary networks such as the IP
Multimedia Subsystem (IMS) and Policy and Charging
Control (PCC). IMS facilitates offering operator-controlled
IP services such as Voice over IP (VoIP), while PCC
facilitates charging and control of QoS. Main PCC nodes
include the Policy and Charging Rules Function (PCRF) and
Policy and Charging Enforcement Function (PCEF). P-GW
usually acts as PCEF from the standard’s perspective.
GERAN/
UTRAN
Core
Evolved Packet Core (EPC)
E-UTRAN
P-GW
S-GW
MME
HSS
SGSN
AAA
Auxiliary
Networks
(IMS and PCC)
Internet
Main PCC Elements: PCRF and PCEF
(PCEF typically implemented by P-GW)
9

In the E-UTRAN architecture, each eNB is now responsible
for all of the functions that used to be divided between the
Node Bs and the RNC. These include:
• Implementation of all of the Layer 1 (Physical Layer)
and Layer 2 sub-layers (Medium Access Control,
Radio Link Control and Packet Data Convergence
Protocol) as well as Radio Resource Control,
• Admission control,
• Allocation and management of all radio resources,
• Control and processing of RF measurements, and
• Control of mobility while in the connected state.
In the EPC, the MME is responsible for high-level security
functions (such as authentication) and manages mobility
while in the idle state. It also determines the
characteristics of the EPS bearer based on the requested
service and QoS requirements.
The S-GW acts as an anchor point for the EPS bearer
allowing traffic to flow seamlessly between the UE and the
network during inter-cell handovers.
The P-GW is similar to a Mobile IP Home Agent (HA)
allocating IP addresses for the UEs and performing any
necessary packet filtering for any necessary firewall and
packet routing functions.
Evolved Packet Core
Functional Split
eNB
PHY
MAC
RLC
PDCP
RRC
Resource
Allocation
Measurement
Configuration
Admission
Control
Connection
Mobility
RB Control
Inter-cell RRM
MME
Serving Gateway PDN Gateway
EPS Bearer
Control
Idle State
Mobility
NAS Security
Mobility
Anchoring Packet Filtering
UE IP Address
Allocation
S1
10

Air Interface
11

The LTE-Uu air interface is divided into a user plane (for
user traffic) and a control plane (for signaling). The user
plane supports the exchange of packets over the radio
bearer between the UE and the serving eNB and is divided
into the following layers and sublayers.
Layer 1 (the PHY or Physical Layer) is responsible for the
actual radio transmission and includes coding for forward
error correction, modulation, bit interleaving, scrambling
and other functions needed to minimize errors over the
radio link. The PHY Layer also manages the operation of
Hybrid ARQ (HARQ), which provides a fast error-correction
mechanism through incremental redundancy.
Layer 2 is divided into the following sublayers:
• The Medium Access Control (MAC) sublayer handles
the scheduling of uplink and downlink resources and
determines the transport format to be used. It also
takes care of multiplexing packets into a single
transmission and inserts padding bits as required.
• Radio Link Control (RLC) performs segmentation and
concatenation to optimize the use of the available
resources, and tracks which packets were sent and
received. Duplicate packets are discarded, out-of-
sequence packets are reordered, and missing
packets are retransmitted.
• Packet Data Convergence Protocol (PDCP)
implements Robust Header Compression (ROHC) and
any required ciphering (encryption) functions.
The LTE-Uu user plane terminates at the eNB.
LTE-Uu User Plane
eNB
PHY
MAC
RLC
PDCP
UE
PHY
MAC
RLC
PDCP
LTE-Uu
Sublayer Key Functions
PDCP Header compression, ciphering
RLC Duplicate and out-of-order detection, segmentation and
concatenation, missing packet retransmission
MAC Packet format selection, scheduling, multiplexing,
padding
PHY OFDMA/SC-FDMA, Coding, modulation, interleaving,
scrambling, HARQ
12

The LTE-Uu control plane carries the signaling necessary
to set up and manage the radio bearer. In addition to the
sublayers described for the user plane, the control plane
also includes Radio Resource Control (RRC), which
handles:
• Broadcasting of system information blocks (SIBs) and
other overhead information,
• Paging of idle UEs,
• Setting up and managing signaling radio bearers,
• Setting up and managing traffic radio bearers,
• UE measurement control and report processing, and
• Handover control and coordination.
In addition, the control plane also carries Non-Access
Stratum (NAS) signaling destined for the core network.
The RRC terminates at the eNB, while NAS signaling is
carried transparently across the eNB to the MME.
LTE-Uu Control Plane
eNB
PHY
MAC
RLC
PDCP
RRC
UE
PHY
MAC
RLC
PDCP
RRC
NAS
MME
NAS
LTE-Uu
Sublayer Key Functions
NAS Non-Access Stratum signaling
RRC System information broadcast, paging, RRC connection
and radio bearer management, handover, UE
measurement control
13

LTE Protocols and
Interfaces
14

The S1 interface connects the E-UTRAN to the EPC. The
S1 is split into a control plane (C-plane), called the S1-
MME, and a user plane (U-plane), called the S1-U. The S1-
MME carries signaling messages between the eNB and
the MME, while the S1-U carries user traffic between the
eNB and the S-GW.
The S1-MME carries S1 Application Protocol (S1AP)
messages, using Stream Control Transmission Protocol
(SCTP) over IP to provide guaranteed data delivery; each
SCTP association between an eNB and a MME can
support multiple UEs. S1AP messages are used for EPS
bearer setup and release procedures, handover signaling,
paging, and NAS signaling transport.
The S1-U consists of a GPRS Tunneling Protocol – User
Plane (GTP-U) tunnel running on the User Datagram
Protocol (UDP), which provides non-guaranteed data
delivery. One GTP tunnel is established for each radio
bearer in order to carry user traffic between the eNB and
the selected S-GW.
The S1-U also implements Differentiated Services Code
Point (DSCP) marking of packets. The 6-bit DSCP value
assigned to each IP packet identifies a pre-determined
level of service and a corresponding packet priority, which
is used to implement the appropriate QoS behavior for the
user’s service.
IP
S1 Interface
MME
S-GW
eNB
S1-
MME
• One logical S1AP
connection per UE
• Multiple UEs supported
via a single SCTP
association
S1AP
SCTP
IP
Data Link Layer
Physical Layer
Physical Layer
S1-U
GTP-U
UDP
IP
Data Link Layer
Physical Layer
GTP-U
• One or more GTP
tunnels per user
• DSCP marking is
supported for QoS
15

The X2 interface allows eNBs to communicate directly with
each other and coordinate their activities. Like S1, the X2
is split into separate C-plane and U-plane functionality.
The X2 C-plane carries X2 Application Protocol (X2AP)
messages between eNBs, and also uses SCTP for reliable
delivery. X2AP is used to manage intra-LTE (inter-eNB)
mobility and handovers, UE context transfers, inter-cell
interference management, and various error-handling
functions. The X2 U-plane is very similar to S1-U, and uses
GTP-U to tunnel user traffic between eNBs
X2 Interface
IP
eNB
eNB
eNB X2
X2 X2
• Multi-cell radio resource
management
• Handover
• Load management
• User packet tunneling
C-plane
X2AP
SCTP
IP
Data Link Layer
Physical Layer
Physical Layer
U-plane
GTP-U
UDP
IP
Data Link Layer
Physical Layer
GTP-U
16

LTE Devices
17

Five different categories of LTE devices have been defined
for Release 8. The categories define some of the basic
physical capabilities of the UE.
The categories differ primarily in the peak data rate that
each supports, ranging from 5 Mbps on the uplink and 10
Mbps on the downlink for a Category 1 device, to 75 Mbps
on the uplink and 300 Mbps on the downlink for a
Category 5 device. All LTE UE categories can support the
same values for system bandwidth and duplexing
schemes. Support for multiple transmit antennas only
applies to the downlink, and that only a Category 5 LTE UE
supports 64QAM modulation on the uplink. The support
for FDD and TDD is independent of the UE category and is
a function of the specific frequency bands supported by a
UE. The differences among the categories 2, 3, and 4 are
the peak data rates due to processing capabilities and
buffer sizes of UEs. The data rates shown in the table are
calculated at the boundary between the physical layer and
the Medium Access Control (MAC) Layer in the air
interface protocol stack. Note that the actual peak data
rates in a given deployment may be lower than shown
here, due to the network configuration.
UE category 1 has 1 receive antenna. UE categories 2, 3,
and 4 have 2 receive antennas. Finally, UE category 5 has
4 receive antennas. Hence, UE category 1 does not
support traditional MIMO, whereas UE categories 2, 3,
and 4 can support (2x2) MIMO, and UE category 5 can
support (4x4) MIMO.
Release 8 LTE UE Categories
UE
Category
Bandwidth
(MHz)
DL
MIMO
Duplexing Modulation
Approximate Data
Rates (Mbps)
UL DL UL DL
1
1.4, 3, 5,
10, 15, 20
N/A
FDD,
H-FDD,
TDD
(Based on
supported
frequency
bands)
QPSK,
16QAM QPSK,
16QAM,
64QAM
5 10
Up to
2x2*
2 25 51
3 51 102
4 51 150
5
Up to
4x4*
QPSK,
16QAM,
64QAM
75 300
*Note: Multiple transmit antennas are
supported on the downlink only.
18

A typical UE must go through a number of steps before it
can even request a service, and setting up and managing
the service itself requires several additional steps.
When a UE arrives or powers up in an E-UTRAN, it must:
1. Acquire the network by scanning for an eNB,
synchronizing with the network, and listening for
system information over the broadcast channels.
2. Establish a signaling connection in order to
communicate with the eNB.
3. Request an attach to the core network in order to
register and receive service.
4. Pass authentication with the core network and
exchange information about security algorithms and
keys.
5. Establish a default EPS bearer with the default
Packet Data Network (Example: Internet PDN or IMS
PDN) and obtain an IP address to provide always-on
experience to the user.
6. Set up default EPS bearers with each of the other
PDNs if needed and set up one or more dedicated
EPS bearers if the QoS of services cannot be satisfied
by the default EPS bearer.
7. Exchange packets with the network after obtaining DL
and UL resources from the eNodeB.
8. Follow the handover instructions from the eNodeB
when the RF environment changes to maintain the
best possible radio link.
9. Enter the idle mode when the radio resources are no
longer required due to lack of data activity.
10. Carry out detach operation in case of power-off.
In case of Detach, the UE loses all EPS bearers and the IP
address. The EPS no longer knows the location of the UE.
Life of an LTE Mobile
UE
1. DL network
acquisition upon
UE power-up
2. RRC signaling
connection setup
3. Attach Request from
UE to MME
4. Authentication and
security
5. Default bearer setup and IP
address allocation for always-on
experience
6. Setting up of additional default
and/or dedicated EPS bearers
(if needed)
7. DL and UL resource
allocation by eNB
scheduler
8. UE-assisted network
controlled hard
handover
9. Idle mode for UE in
absence of data activity
10. Power-off
detach
19

Summary
• Key goals of 3GPP evolution are:
– Increased data rates and reduced latency,
– Higher capacity and better cell coverage, and
– Reduced cost to users and operators.
• New interfaces and network architectures of LTE include the:
– Evolved air interface based on OFDM and MIMO,
– Evolved radio networks (the E-UTRAN with only eNBs),
– IP-based Evolved Packet Core (EPC), and
– S1 and X2 interfaces for the LTE RAN.
• Key features of the air interface are:
– OFDMA for improved spectral efficiency,
– Support for multiple-antenna techniques,
– Scalable bandwidth (1.4 to 20 MHz), and
– Data rates up to 300 Mbps in the DL and up to 75 Mbps in the UL.
20

Review Questions
1. Which functions does the eNB perform?
2. What path does user traffic take through the E-
UTRAN and EPC?
3. What does the eNB do with NAS messages?
4. Highlight the top three key differences among the
UE categories.
5. Give examples of functions that occur before the
UE can do data transfer.
21

22

2 | LTE Air Interface Essentials
Chapter 2:
LTE Air Interface
Essentials
23

References:
[1] 36.300 – E-UTRA and E-UTRAN Overall Description
(Stage 2)
[2] 36.211-36.214: Physical Layer related documents
Objectives
• Explain the key characteristics of the LTE
downlink and uplink
• Describe the structure of LTE frames and
resource blocks
• Identify the physical, transport and logical
channels in LTE and how they relate to one
another
• Specify how multiple antennas are used in LTE
24

OFDMA and SC-FDMA
25

The E-UTRA uses Orthogonal Frequency Multiple Access
(OFDMA) as its fundamental transmission technology on
the downlink in order to take advantage of the benefits
OFDMA offers:
• High Spectral Efficiency: OFDMA makes better use of
the available spectrum than CDMA technologies
providing significantly higher data rates for a given
bandwidth.
• Robust Against Multipath Interference: The relatively
large OFDM symbol time means that the short delay
spreads typically found in wireless networks have
minimal impact on the quality of the signal. Use of a
Cyclic Prefix (CP) further reduces the effect of
multipath interference.
• Support for MIMO: OFDMA inherently lends itself to
implementing Multiple Input Multiple Output (MIMO)
and other multiple-antenna techniques.
• Resource Allocation: Users can be allocated
resources in both the time domain (symbols) and
frequency domain (subcarriers). This provides a
tremendous level of flexibility for the eNB to maximize
the effective use of the available resources.
• Reduced Receiver Complexity: The use of Fast Fourier
Transform (FFT) and Inverse Fast Fourier Transform
(IFFT) processing greatly simplifies the design of the
transmitter and receiver.
LTE Downlink
UE
eNB
UE
UE
OFDMA
High spectral
efficiency
Robust against
multipath
interference
Support for
MIMO
Time and frequency
resource allocation
26

The effective management of transmit power, complexity,
and cost are key determining factors for handset design.
To that end, LTE has chosen a somewhat different
solution for the uplink, Single-Carrier Frequency Division
Multiple Access (SC-FDMA). Although very similar in nature
to OFDMA, SC-FDMA has a number of unique
characteristics which are particularly attractive on the UE
side.
• Peak-to-Average Power Ratio (PAPR) Reduction: Large
numbers of independent symbols are not summed to
create the transmitted signal, which greatly reduces
the PAPR experience by the UE.
• Lower Maximum Power Requirement: Because of the
lower PAPR, less power backoff is needed, and a
power amplifier with a lower rating (and lower cost)
can be used.
• Better Cell-Edge Performance or Large Cells:
Reduction in PAPR can be exploited to improve the
link budget, or to improve cell-edge performance.
• Complexity: SC-FDMA needs more processing blocks
at the transmitter and receiver.
LTE Uplink
eNB
SC-FDMA
Reduced Peak-to-
Average Power
Ratio (PAPR)
Better Cell-edge
Performance or
Larger Cell Sizes
Increased
Complexity of
Transmitter &
Receiver
UE
UE
UE
27

A major challenge associated with OFDM is the high Peak-
to-Average Power Ratio (PAPR) of the transmitted signal.
The very high peaks are a direct consequence of the IFFT
summing multiple independent symbols, which are all
integer number of cycles over the symbol time; whenever
the symbols add constructively, the result is a peak of
power. Signals with a high PAPR either require highly
linear power amplifiers (which are expensive) or must be
clipped (which introduces distortion).
The transmitters in an SC-FDMA system also use multiple
subcarriers to transmit information symbols; however,
they transmit the symbols sequentially rather than in
parallel. Relative to OFDMA, this approach reduces the
fluctuations in the transmitted waveform.
SC-FDMA and PAPR
Peak
Average
High PAPR
OFDMA
Peak
Average
Low PAPR
SC-FDMA
28

In OFDMA systems the transmitter uses multiple
subcarriers to modulate the information in parallel, and
then sends them through an Inverse Fast Fourier
Transform (IFFT), which is a weighted summation of these
independent symbols.
An SC-FDMA system uses a single carrier to modulate the
information symbols sequentially. On the transmitter side,
the additional block of the SC-FDMA is a Discrete Fourier
Transform (DFT). This block transforms a time-domain
modulated symbol stream into the frequency domain.
Then, the frequency domain information is mapped to a
wider range (spreading) and goes through an IFFT, which
transforms the frequency domain information back into
the time domain. SC-FDMA is also referred to as DFT-
spread OFDMA. Since the individual symbols are
serialized and distributed across multiple subcarriers, the
PAPR issues associated with traditional OFDMA
transmissions are reduced.
OFDMA vs. SC-FDMA
10010111
Digital
Modulation
Channel
Mapping
IFFT
f1
f2
f3
.
.
fn
OFDMA
Modulation
symbols
10010111
Digital
Modulation
DFT
Spreading
IFFT
f1
f2
f3
.
.
fn
SC-FDMA
DFT symbols
29

LTE Frame Structure
30

Although all 4G systems are based on OFDM technology,
they are not identical. LTE has laid out a set of system
parameters and capabilities that define its unique
characteristics.
• Bandwidth: LTE is a scalable OFDMA system,
supporting channel bandwidths of 1.4, 3, 5, 10, 15,
and 20 MHz.
• Subcarrier Spacing: LTE subcarriers are spaced
exactly 15 kHz apart.
• Antennas: LTE supports multiple-antenna systems
with as many as four transmit and four receive
antennas (4x4 MIMO).
• Frame Timing: A single frame is 10 ms long.
• Cyclic Prefix: Two values for the CP have been
defined. The Normal prefix is 5.2 μs for the first
symbol in a slot and 4.69 μs for the remaining
symbols, while the Extended prefix is 16.67 μs for all
symbols. The useful symbol time is 66.67 μs,
regardless of the CP value.
• Duplexing: Most LTE systems will use Frequency
Division Duplexing (FDD), where separate paired
uplink and downlink channels can be used
simultaneously. Half-Duplex FDD (H-FDD) is also
defined, using paired channels but alternating
transmissions, while Time Division Duplex (TDD) uses
a single channel for both downlink and uplink
transmissions.
• Coding: Depending on the content being sent, either
convolutional coding or turbo coding may be used to
protect the data. Convolutional coding adds less
delay, while turbo coding is more robust.
LTE Design Parameters
Bandwidth
20 MHz
15 MHz
10 MHz
5 MHz
3
1.4
Subcarrier Spacing
15 kHz
Antennas
Up to 4x4 MIMO
Frame Timing
n-1 n n+1
10 ms
Cyclic Prefix
CP Symbol
CP Symbol
16.67 μs
5.2 or 4.69 μs
Normal
Extended
66.67 μs
Duplexing
FDD
H-FDD
TDD
Coding
Convolutional Turbo
0101
000111000111
0101
100110001011
Modulation
QPSK 16-QAM 64-QAM
Multiple Access
OFDMA
SC-FDMA
31

• Modulation: Three modulation schemes are
supported for data transmission. Quadrature Phase
Shift Keying (QPSK) carries two bits per symbol, while
16 Quadrature Amplitude Modulation (16QAM) and
64QAM carry four bits per symbol and six bits per
symbol, respectively. LTE uses Adaptive Modulation
and Coding (AMC), adjusting the coding rates and
modulation scheme dynamically.
• Multiple Access: LTE uses traditional OFDMA on the
downlink, but uses a variation called Single Carrier
FDMA (SC-FDMA) on the uplink to reduce the Peak-to-
Average Power Ratio (PAPR).
LTE Design Parameters (Continued)
Bandwidth
20 MHz
15 MHz
10 MHz
5 MHz
3
1.4
Subcarrier Spacing
15 kHz
Antennas
Up to 4x4 MIMO
Frame Timing
n-1 n n+1
10 ms
Cyclic Prefix
CP Symbol
CP Symbol
16.67 μs
5.2 or 4.69 μs
Normal
Extended
66.67 μs
Duplexing
FDD
H-FDD
TDD
Coding
Convolutional Turbo
0101
000111000111
0101
100110001011
Modulation
QPSK 16-QAM 64-QAM
Multiple Access
OFDMA
SC-FDMA
32

Some key OFDMA/SC-FDMA transmission parameters are
provided in this table. LTE is a scalable system so the
subcarrier spacing (15 kHz) is the same regardless of the
amount of spectrum. A 10 MHz system has a the FFT size
of 1024 and includes 50 resource blocks (RBs) (50*12 =
600 subcarriers) for assignment to users. One RB consists
of 12 subcarriers. The used spectrum is thus
approximately (600 * 15 kHz= 9 MHz), with the remaining
(10 MHz – 9 MHz = 1 MHz) being the total guard band.
Out of the 1 MHz total guard band, 0.5 MHz guard band is
on the left and 0.5 MHz guard band is on the right of the
transmission bandwidth of 9 MHz. The guard band
minimizes interference between the LTE system and the
adjacent system. Since a 10-ms radio frame has 10
subframes, each subframe is 1 ms long. Furthermore, two
slots exist per subframe, yielding a 0.5-ms slot. When a
normal cyclic prefix (CP) is used, seven symbols exist in a
slot, and, when an extended CP is used, six symbols exist.
The data channels support the modulation schemes of
QPSK, 16QAM, and 64QAM for flexibility and higher
throughput. Control signaling uses BPSK and QPSK for
reliability.
LTE Transmission Parameters
Parameters Values
Bandwidth (MHz) 1.4 3 5 10 15 20
Sub-frame Duration 1 ms
Subcarrier Spacing 15 kHz
FFT Size 128 256 512 1024 1536 2048
Used Subcarriers 72 180 300 600 900 1200
Resource Blocks 6 15 25 50 75 100
OFDM Symbols per Slot 7 or 6
Modulation Schemes
BPSK, QPSK (Physical Layer Signaling)
QPSK, 16QAM, 64QAM (Data and RRC Signaling)
Error Coding
Rate 1/3 Convolutional (Physical Layer Signaling)
Rate 1/3 Turbo (Data and RRC Signaling)
33

The duration of one LTE radio frame is 10 ms. One frame
is divided into 10 subframes of 1 ms each, and each
subframe is divided into two slots of 0.5 ms each. Each
slot contains either six or seven OFDM symbols,
depending on the Cyclic Prefix (CP) length. The useful
symbol time is 1/15 kHz= 66.6 mircosec. Since normal
CP is about 4.69 microsec long, seven OFDM symbols can
be placed in the 0.5-ms slot as each symbol occupies
(66.6 + 4.69) = 71.29 microseconds. When extended CP
(=16.67 microsec) is used the total OFDM symbol time is
(66.6 + 16.67) = 83.27 microseconds. Six OFDM symbols
can then be placed in the 0.5-ms slot. Frames are useful
to send system information. Subframes facilitate resource
allocation and slots are useful for synchronization.
Frequency hopping is possible at the subframe and slot
levels.
LTE Frame Structure
UE
eNB
Frame n Frame n+1 Frame n+2
Frame n-1
10 ms
Subframe 0 Subframe 1 Subframe 2 Subframe 9
Slot 0 Slot 1
1 ms
0.5 ms
34

In LTE, radio resources are allocated in units of Physical
Resource Blocks (PRBs). PRB spans 12 subcarriers and
one slot. If the normal CP is used, a PRB will contain 12
subcarriers over seven symbols. If the extended CP is
used, the PRB contains only six symbols. The UE is
specified allocation for the first slot of a subframe. There
is implicit allocation for the second slot of the subframe.
For example, if the eNB specifies one RB as the resource
allocation for the UE, the UE actually uses two RBs, one
RB in each of the two slots of a subframe. When
frequency hopping is turned on, the actual PRBs that carry
the UE data can be different in the two slots. In a 10 MHz
spectrum bandwidth, there are 600 usable subcarriers
and 50 PRBs.
Physical Resource Blocks
Slot 0 Slot 1
12
Subcarriers
PRB
7 Symbols
Normal CP
PRB
12
Subcarriers
6 Symbols
Extended CP
35

Exercise
True or False?
1. OFDMA is used in the DL and SC-FDMA is used
in the UL.
2. SC-FDMA helps reduce PAPR.
3. SC-FDMA needs more processing blocks at the
transmitter and receiver compared to OFDMA.
4. One PRB has a total of 600 sub-carriers.
5. One PRB has a total of 72 or 84 modulation
symbols.
36

LTE Channels and
Signals
37

The relationships between the various physical, transport
and logical channels are illustrated here. Note that some
physical channels carry only control information and are
not linked to transport channels.
Logical channels carry signaling and data between the UE
and the eNB are the:
• Broadcast Control Channel (BCCH): Contains
broadcast messages.
• Paging Control Channel (PCCH): Carries page
messages.
• Dedicated Control Channel (DCCH): Contains control
messages to and from specific UEs.
• Dedicated Traffic Channel (DTCH): Contains data and
some call-related signaling messages to and from
specific UEs.
• Common Control Channel (CCCH): Carries control
information and service requests to and from idle
UEs.
Transport channels carry following information in the
downlink and uplink are the:
• Broadcast Channel (BCH): Transmits broadcast and
system overhead messages.
• Downlink Shared Channel (DL-SCH): Carries user
traffic, signaling messages, page messages, and
system information in the downlink.
• Paging Channel (PCH): Carries a page message.
• Uplink Shared Channel (UL-SCH): Carries user traffic
and signaling messages in the uplink.
• Random Access Channel (RACH): Controls the use of
the PRACH by choosing parameters such as random
backoff.
PBCH
PCFICH (CFI)
DTCH
PCCH
PCH
DCCH
DL-SCH
BCCH
BCH
PDSCH
PDCCH (DCI) CCCH
PHICH (HI)
PUCCH (UCI)
DTCH
CCCH RACH PRACH
DCCH PUSCH (UCI)
UL-SCH
Logical Channel
Transport Channel
Physical Channel
D
O
W
N
L
I
N
K
U
P
L
I
N
K
Logical Channel Transport Channel Physical Channel
Channels: Mapping and Their Use
38

Physical channels serve the following purposes:
• The PDCCH carries the Downlink Control Information
(DCI), which includes resource allocations and the
corresponding modulation and coding schemes,
power control commands, channel quality requests,
and other commands for the UE.
• The PCFICH carries the Control Format Indicator (CFI),
which indicates how many symbols the PDCCH
occupies in each subframe.
• The PHICH carries the Hybrid ARQ Indicator (HI),
which is used to ACK and NACK ongoing uplink data
transmissions.
• The PUCCH carries the Uplink Control Information
(UCI), which includes scheduling requests, channel
quality reports, and Hybrid ARQ ACKs and NACKs for
downlink transmissions.
Channels: Mapping and Their Use
(Continued)
PBCH
PCFICH (CFI)
DTCH
PCCH
PCH
DCCH
DL-SCH
BCCH
BCH
PDSCH
PDCCH (DCI) CCCH
PHICH (HI)
PUCCH (UCI)
DTCH
CCCH RACH PRACH
DCCH PUSCH (UCI)
UL-SCH
Logical Channel
Transport Channel
Physical Channel
D
O
W
N
L
I
N
K
U
P
L
I
N
K
Logical Channel Transport Channel Physical Channel
39

We clear the confusion that sometimes obscures the
difference between Physical channels and Physical signals
by noting that physical Channels carry upper-layer
information and physical signals do not.
Physical Channels carry, for example, traffic channels,
which can carry email or enable an FTP transfer. Physical
signals, on the other hand, have nothing in them from
outside the Physical Layer itself. A pilot reference or a
sounding reference are examples of a physical signal.
• Reference Signals: Reference signals (also known as
pilots) provide a known or predictable pattern that
allows the UE to decode the physical channels and
estimate downlink channel conditions. Reference
signals may be cell-specific (common to all UEs) or
UE-specific.
• Synchronization Signals: Synchronization signals
allow UEs to detect and identify cells during initial
system acquisition and provide an initial timing
reference.
The UE also provides reference or pilot signals to allow the
network estimate uplink channel conditions and to
coherently demodulate its transmissions.
Physical Signals
UE eNB
Reference Signals
(Channel Estimation and
Coherent Demodulation)
Synchronization Signals
(Power-up Synchronization)
Reference Signals
• Demodulation Reference Signal (DMRS)
• Sounding Reference Signal (SRS)
40

In every subframe, the first one, two, or three OFDM
symbols contain one or more PDCCHs, which carry
scheduling assignments and other control information.
The exact number of OFDM symbols used is specified in
the PCFICH, which appears in symbol 0 of the even-
numbered slots (in other words, the first slot of each
subframe). The PHICH may also appear in the first symbol,
as required. The remaining symbols in the two slots
contain the PDSCHs, which carry user data and signaling
information to specific users.
Downlink Resource Mapping
0 1 2 3 4 5 6 0 1 2 3 4 5 6
Slot n Slot n+1
PDCCHs
Resource
Blocks
PHICH
PCFICH
PDSCH (User A)
PDSCH (User B)
PDSCH (Paging)
PDSCH (System Broadcast - SIBs)
41

In LTE, one reference signal is transmitted per antenna
port to estimate the channel response for each antenna
on the downlink. The location of each reference signal
within a resource block depends on the number of
antennas and the slot within the subframe; the reference
signals for a four-antenna system (antenna ports 0, 1, 2
and 3) are illustrated here.
Cell-specific reference signals play an important role in
channel estimation and system acquisition. During
channel estimation, the receiver compares the received
symbols with known reference symbols and estimates the
channel response for channel equalization purposes.
DL Reference Signals
Slot n Slot n+1
Subcarriers
0
0
1
0
1
2
1
3
2
3
2 3
1
4
0
1
0
5 6
0
0
1
0
1
3
1
2
3
2
2 3
1
4
0
1
0
5 6
42

In the uplink, the Physical Uplink Control Channel (PUCCH)
occupies one or more resource blocks at each end of the
system bandwidth and performs frequency hopping
between the first and second slots within a subframe. The
resource blocks between the PUCCHs are used for
Physical Uplink Shared Channel (PUSCH), and may do
frequency hopping between slots. The PUSCH carries
user-specific traffic and/or RRC signaling messages. The
Physical Random Access Channel (PRACH) occupies six
resource blocks over one or more consecutive subframes,
adjacent to the PUCCH. The frequency at which the
PRACH appears in the UL frame is configured as
broadcast information in the System Information Block
(SIB).
Uplink Resource Mapping
0 1 2 3 4 5 6 0 1 2 3 4 5 6
Slot n Slot n+1
PUCCH
Resource
Blocks
PUCCH
PRACH
PUSCH
PUCCH
PUCCH
43

Exercise
True or False?
1. The PCFICH informs the UEs in a cell about the
number of OFDM symbols used by the PDCCHs.
2. The PDCCH can allocate DL resources to a UE;
however, it cannot allocate UL resources to the
UE.
3. The PDSCH can carry traffic and UE-specific
signaling messages.
4. The PHICH enables the UE to improve UL
throughput by carrying ACK/NACK.
44

Multiple-Antenna
Techniques
45

This slide shows the multiple-antenna techniques used in
LTE. The underlying assumption is that the signals out of
different antennas experience different fading.
• Diversity: Diversity techniques increase link
robustness and thus coverage, including receive
diversity and transmit diversity. With transmit
diversity, the same data stream is sent to the same
user and experiences independent fading. So, the
signal quality is enhanced by combining them.
• Multiple-Input Multiple-Output (MIMO): Also known as
spatial multiplexing, different data streams are sent
from different antennas. If the streams are sent to
the same user, it is called Single-User MIMO (SU-
MIMO) and can increase the data rate for that user. If
the streams are sent to different users, it is called
Multi-User MIMO (MU-MIMO) and can increase
capacity for the cell.
• Beamforming: Spatial Division Multiple Access
(SDMA) is the most complicated beamforming and
the theoretical foundation of MU-MIMO. So, in
practice, MU-MIMO is equivalent to SDMA and can be
viewed as MIMO plus Beamforming (BF). For simple
beamforming, it can be implemented as a special
case of SU-MIMO, where a single stream is sent out.
Multiple-Antenna Techniques in LTE
Multiple-Antenna Techniques
Diversity Beamforming
MIMO/Spatial Multiplexing
Transmit
Diversity
Receive
Diversity
SU-MIMO
(Single User
MIMO)
SDMA
(Spatial Division
Multiple Access)
MU-MIMO
(Multi-User
MIMO)
=
Special Case
of SU-MIMO
46

Two basic forms of diversity are receive diversity and
transmit diversity.
• Receive Diversity: This form of diversity is
implemented at the receiver. Multiple antennas are
used at the receiver. The BS transmits just one signal,
but multiple copies of the same signal are received
because of the multiple antennas. These signals
experience different fading characteristics, and the
probability that all signals experience a fade at the
same time is low. Hence, the quality of the combined
overall signal is likely to be good. The advantage of
receive diversity is better performance compared to a
single receive antenna. On the other hand, the
disadvantage is increased cost at the terminal when
receive diversity is implemented at the terminal.
• Transmit Diversity: This form of diversity is
implemented at the transmitter. The transmitter
transmits a signal, possibly in different forms, from
multiple antennas at the same time. With transmit
diversity, multiple copies of the same data stream are
sent to the same user and each stream experiences
independent fading. Thus, multiple copies of
essentially the same signal are received at the
terminal, providing diversity benefits, as is the case
with receive diversity. However, transmit diversity has
an added benefit, it can be implemented at the BS
without requiring multiple antennas at the MS.
Generic Diversity Techniques
Receive diversity
Transmit diversity
BS
• Improved link robustness (same stream to same user)
• Better coverage
Multiple antennas at
the receiver to leverage
the signal variation in
space by suitably
combining the multiple
copies of the signal
Multiple antennas at the
transmitter; intelligent use of
space, frequency and time
(Space Frequency Block Coding
– SFBC) to obtain multiple
copies of the signal at the
receiver
47

In Single-User MIMO (SU-MIMO), also known as Spatial
Multiplexing (SM), each antenna carries a separate data
stream on each frequency assigned to the user. Each
receiver picks up the combined signal on each frequency,
containing the sum of all symbols sent in each symbol
time. The multiple copies of the same received signal,
along with the UE’s knowledge of the MIMO matrix
channel (derived from reference signals from each
transmit antenna), allow the UE to extract each of the
original symbols.
This diagram illustrates a 2x2 MIMO system, with two
transmit antennas and two receive antennas. The signal
between transmit antenna 1 and receive antenna 1 is
quantified by the channel response h11. Each of the four
possible paths has its own channel response
characteristics. Accurate channel characterization allows
each transmit antenna to independently deliver a different
data stream, potentially increasing the peak data rates
linearly with the number of transmit antennas; a 2x2
system can double the data rate, while a 4x4 system can
quadruple it.
The net result is a significantly higher data rate, since
each transmit antenna is sending a separate data stream
in parallel using the same frequency and time resources,
at the cost of increased receiver complexity.
Single-User MIMO (SU-MIMO)
Tx
1
Tx
2
Rx
1
Rx
2
A
A
B
-B
A
B
A+B
A-B
Each antenna transmits a
separate data stream
Each receiver detects the
combination of all symbols on
each frequency
h11
h22
h12
h21
48

Beamforming is a signal-processing technique that mimics
older hardware methods that used variable-length cables
going to different antenna elements. The modern signal
processing methods are widely used in electronic antenna
arrays for the directional transmission or reception of
signals. The improvement derived from narrowed and
directional beams, when compared with omnidirectional
transmissions or receptions, is known as the
transmit/receive gain or loss.
Beamforming results in spatial selectivity, which is
achieved by using adaptive or fixed receive/transmit
beam patterns. The beam patterns come from the
different relative phase shifts observed between the
antennas at different points in space. A mobile, for
example, at a certain point relative to two antennas
transmitting the same information at different relative
phase offsets will “see” varying degrees of constructive or
destructive combing of the two waveforms from each of
the two antennas at different points.
When transmitting, a beamformer controls the relative
phase of the signals from each transmitter antenna in
order to create a pattern of constructive and destructive
interference in the wave front at the receiver.
Beamforming Concept
Signal 1
Signal 2
Add in-
phase
signals
A
A
2A
UE
After Beamforming
Power = (2A)2 = 4A2
eNB
In-phase
Antenna
1
Antenna
2
49

MU-MIMO in LTE provides multi-stream transmission to
multiple users simultaneously. This increases the system
throughput. Also, it increases the sum data-rate and
reduces the latency of mobile users as compared to other
multiple access schemes like TDMA. In MU-MIMO inter-
user interference is a major challenge and needs to be
taken care at the transmitter so as to save UE power
consumption and keep them as simple as possible. The
Multi User MIMO (MU-MIMO) theory is based on SDMA
concept, i.e., use of spatial sharing of the channel by the
users. It schedules multiple users in one time frequency
slot. It needs extra hardware but no extra bandwidth.
MU-MIMO: SDMA Implementation
120° cellization
cell B
Radio channels in cell B: 100
cell throughput: 80 Mbps
40 Mbps
40 Mbps
SDMA Beam1 SDMA Beam2
eNB
f1,f2,f3……f100 f1,f2,f3……f100
• Benefit : High throughput
• Drawback : Potential for high interference
50

While MU-MIMO can significantly increase capacity and/or
throughput in the downlink, it can also be applied in the
uplink. The goal of uplink MU-MIMO is to increase uplink
sector throughput with just one transmit antenna at the
UE. Note that the cost of the UE is kept low in this case. If
we start using multiple transmit antennas at the UE, we
would need to worry about the cost of the antennas,
transmit power requirements, processing power, and
complexity.
Consider a system in which we want to implement uplink
(2x2) MU-MIMO. To have two transmit antennas in the
uplink, we will use one antenna from one UE and another
antenna from another UE. The eNB uses two receive
antennas to receive signals from two transmit antennas.
The basic operation of uplink MU-MIMO is similar to that
for downlink MU-MIMO. Basically, one transmit antenna
sends out one data stream, and another transmit antenna
sends out another data stream. Two UEs use the same
radio resource (frequency time slot). In order to use uplink
MU-MIMO, two UEs need to locate in good radio condition.
Uplink MU-MIMO can increase UL sector throughput. For
example, if a system with one transmit antenna and one
receive antenna yields a sector throughput of 10 Mbps in
the uplink, uplink (2x2) MU-MIMO can provide sector
throughout of 20 Mbps in the uplink.
MU-MIMO in the UL
Goals
• Increase UL sector throughput
• Keep terminal cost lower
1 Tx Antenna
UE1
1 Tx Antenna
UE2
Multiple users in the
same radio resource
(frequency time slot)
51

When closed-loop antenna selection is enabled the eNB
tells its UE which of its antennas to use on its PUSCH with
an implicit signaling mechanism within the UL scheduling
grant on the eNB’s DL Control Information (DCI) directed
toward the UE. The DCI includes all kinds of information
useful to the UE: Radio Bearer (RB) assignments,
prescribed hopping sequences (ON or OFF, intra- or inter-
sub-frame frequency hopping in explicit or pre-defined
patterns), the applicable Modulation and Coding Scheme
(MCS), Transmit Power Control (TPC) for the UE, the power
of the Demodulation Reference Signal (DMRS) relative to
the eNB’s carrier power, and the CQI requests. The DCI’s
message includes a 16-bit Cyclic Redundancy Count
(CRC), a type of block coding which is masked with the
UE’s ID (iC-RNTE or Cell-RNTI) and the eNB’s antenna
preferences (antenna port 0 or antenna port 1).
PUSCH and UL Antenna Selection
DCI Format Description
0 • Basic Contents:
UL resource allocation (RBs, hopping, MCS), TPC, DMRS, CQI
Request
• CRC for the DCI:
 16 bits
 Masked with
(i) UE ID (i.e., C-RNTI)
(ii) UE transmit antenna selection mask (0 or 1 for antenna
port 0 or 1)
Antenna Port 0
UE
Antenna Port 1
(DCI)
PDCCH
(Traffic)
PUSCH
eNB
52

Summary
• Characteristics of the LTE Physical Layer are:
– OFDMA in the DL and SC-FDMA in the UL,
– Channel bandwidth of 1.4 to 20 MHz, and
– Resources are assigned per sub-frame.
• LTE supports multiple-antenna techniques, including:
– Transmit and receive diversity,
– Spatial multiplexing (SU-MIMO), and
– Beamforming and Spatial Division Multiple Access.
53

Review Questions
1. Why was SC-FDMA chosen for the uplink?
2. Describe the relationship among sub-frames,
slots, resource blocks (RB), modulation symbols,
and sub-carriers.
3. What is the overhead of reference signals in LTE?
4. How do multiple-antenna techniques improve
coverage and throughput?
5. Which physical channels carry user traffic in the
UL and DL?
6. Which physical channel carries the DL and UL
resource allocations?
54

3 | System Acquisition
Chapter 3:
System Acquisition
55

References:
[1] 3GPP TS 36.211 – E-UTRA Physical channels and
modulation
[2] 3GPP TS 36.213 – E-UTRA Physical layer procedures
[3] 3GPP TS 36.300 – E-UTRA and E-UTRAN Overall
description stage 2
[4] 3GPP TS 36.306 – E-UTRA User Equipment (UE) radio
access capabilities
[5] 3GPP TS 36.321 – E-UTRA Medium Access Control
(MAC) Protocol specification
[6] 3GPP TS 36.331 – E-UTRA Radio Resource Control
(RRC) Protocol specification
Objectives
• Explain the steps involved in system acquisition
• Describe DL synchronization
• Specify the roles played by various signals and
channels in network acquisition
• Illustrate the cell selection procedure
56

Overview of System
Acquisition
57

After power-up, the UE goes through the process of trying
to find an LTE network to make a connection. The eNB
uses several downlink channels to assist the UE with the
overall Network Acquisition process. First, the UE needs to
synchronize with the downlink transmissions of the eNB.
The Primary Synchronization Signal is used to obtain DL
slot timing synchronization. The eNB repeatedly
broadcasts one of three possible 62-bit sequences to help
the UE recognize where slot transmissions begin.
The Secondary Synchronization Signal is used to obtain DL
frame timing synchronization. The eNB repeatedly
broadcasts one of 168 possible 62-bit sequences to help
the UE recognize where frame transmissions begin. A
different Secondary Synchronization Signal sequence is
transmitted in 2 sub-frames 5 ms apart in every frame.
This difference is used to identify the beginning of the
frame.
Both the primary and secondary synchronization signals
are transmitted on the center 62 subcarriers, and
together identify the cell using one of 504 possible
Physical Layer Cell Identities.
System information is periodically broadcast by all LTE
eNBs. An important system information message called
the Master Information Block (MIB) is broadcast every 40
ms by the eNB on the Physical Broadcast Channel (PBCH).
The MIB contains a few very important LTE system
parameters that are essential in system acquisition.
eNB
UE
DL Physical
Channels/Signals
Purpose
Primary Sync Signal Slot Synchronization
Secondary Sync Signal Frame Synchronization
Physical Broadcast
Channel
Master Information
Block (MIB)
Physical Control Format
Indicator Channel
Amount of Resources
Consumed by PDCCHs
Physical Downlink Control
Channel
UL Power Control, UL and
DL Resource Allocations
Physical Downlink Shared
Channel
DL Traffic, Common and
UE-specific Signaling,
Paging
Downlink Channels
58

The Physical Control Format Indicator Channel (PCFICH) is
used by the eNB to inform the UEs about the number of
OFDM symbols used for the PDCCH.
The Physical Downlink Control Channel (PDCCH) is the
channel used by the eNB to send control information to
the UE. The PDCCH channel carries DL resource allocation
information, UL scheduling grants and Transmit Power
Control (TPC) commands for the UE.
The Physical Downlink Shared Channel (PDSCH) is used to
carry user data, paging and control signals and broadcast
information.
Downlink Channels (Continued)
eNB
UE
DL Physical
Channels/Signals
Purpose
Primary Sync Signal Slot Synchronization
Secondary Sync Signal Frame Synchronization
Physical Broadcast
Channel
Master Information
Block (MIB)
Physical Control Format
Indicator Channel
Amount of Resources
Consumed by PDCCHs
Physical Downlink Control
Channel
UL Power Control, UL and
DL Resource Allocations
Physical Downlink Shared
Channel
DL Traffic, Common and
UE-specific Signaling,
Paging
59

The UE performs the functions shown in the slide in
sequence during initial network acquisition. With the
completion of the cell selection procedure the UE will have
downlink synchronization with the eNB. Following the
power-up, the UE undergoes a series of hardware tests to
verify the integrity of memory and other peripherals. It
selects a frequency band to acquire an LTE system based
on its configured list. At this point, the UE still has no
knowledge of any operator’s presence. It simply tries to
acquire the network.
The UE scans for the sync signals to acquire frequency
and time synchronization. Once the UE acquires primary
and secondary sync signal information, it gains knowledge
about both synchronization and physical cell identity. Now,
the UE is ready to acquire the master information block
from the PBCH to determine the actual DL channel
bandwidth.
Now, the UE decodes the information from the PCFICH
and reads the PDCCH to find the system information
resources on the shared channel (PDSCH).
Then, the UE decodes the System Information Broadcast
(SIB) messages to acquire the PLMN ID of the network. If
the PLMN ID of the network matches with the PLMN ID list
of the UE, then UE runs the cell selection algorithm. If cell
selection is successful, then UE tries to acquire UL
synchronization through the initial access procedure, else
the UE acquires another LTE cell and restarts the process
again.
Process of Network Acquisition
UE Power-up
Downlink Synchronization
and Determination of
Physical Cell ID
UE Ready for Initial Access
UE Frequency and
Time Synchronized
Acquire Another
LTE Cell
System Bandwidth
Known
Shared Channel Resource Acquisition
(PDCCH Processing)
Control Format Indicator Acquisition
(PCFICH Processing)
MIB Acquisition
(PBCH Processing)
Retrieval of SIBs
(PDSCH Processing)
PLMN ID, Cell Selection
Criteria, Other Cell Info
Obtained
PDCCH location Known
Locations of SIBs Found
Cell Selection
Successful?
No
Yes
60

Processing of
Synchronization
Signals and PBCH
61

The UE performs the functions shown in the slide during
Physical Layer Cell Identity (PCI) acquisition. After
completion of power-up tests, the UE initiates DL
synchronization and the PCI acquisition procedure.
The first step in the process is the frequency acquisition
procedure, where the UE tries to acquire the center 72
subcarriers (72 subcarriers * 15 kHz = 1.08 MHz) of the
DL channel to decode the primary and secondary
synchronization signals. The center 72 subcarriers are
equivalent to the smallest usable channel bandwidth (1.4
MHz) supported by LTE. This mechanism of using only the
smallest possible bandwidth configuration provides a
consistent way for UEs to acquire networks that may be
operating with any of the possible bandwidth
configurations (1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz,
20 MHz).
Next, the UE tries to acquire the primary sync signal to get
slot synchronization. The eNB transmits the
synchronization signals on the center 62 subcarriers. The
primary sync signal broadcasts one of three possible 62-
bit sequences every 5 ms (twice per radio frame) to help
the UE recognize where slot transmissions begin. Using
the primary sync information, the UE tries to acquire the
secondary sync signal to acquire frame synchronization.
The secondary sync signal repeatedly broadcasts one of
168 possible 62-bit sequences to help the UE recognize
where frame transmissions begin. The secondary sync
signal information is also transmitted every 5 ms in the
same slots as the primary synch signal. The secondary
synch signal broadcasts a different sequence format in
each of the 2 slot times per radio frame. This difference is
used to identify the beginning of the frame. Both the
primary and secondary sync signals are transmitted on
the center 62 subcarriers, and together they indicate the
Physical Cell Identity (PCI) (one of 504 possible PCIs) for
the cell.
Downlink Synchronization
Primary Sync Signal
Acquisition
• Frame Synchronization Acquired
• Unique (one of 168) Cell Group ID
Sequence Acquired
• Slot Synchronization Achieved
• Unique (one of three) Primary
Sync Signal Acquired
Physical Cell ID Detection
Combination of
Primary and Secondary Sync
Sequences
Secondary Sync Signal
Acquisition
62

The slide shows the UE frequency acquisition procedure in
LTE. The E-UTRA cell search procedure supports a
scalable overall transmission bandwidth of 1.4 MHz to 20
MHz, corresponding to six or more Resource Blocks (RBs).
Each RB consists of 12 subcarriers per OFDM symbol. The
UE first detects the central part of the spectrum
regardless of receiving bandwidth capability.
In this figure, the cell operates at a 20 MHz bandwidth.
The UE first detects the center frequency of the 20 MHz
spectrum. The primary synchronization and secondary
synchronization signals are carried on 62 subcarriers
centered around the center frequency. They carry 62-bit
sequences that occupy the center 62 subcarriers with five
unused subcarriers on each side. Once the UE detects
primary synchronization and secondary synchronization
signals, it acquires DL timing synchronization. Next, the UE
looks for the Physical Broadcast Channel (PBCH). The
PBCH information is always spread over 72 subcarriers
centered around the center frequency. Now that the UE
has DL frame synchronization, it acquires the PBCH and
reads the MIB. One essential piece of information sent on
the PBCH is the actual DL channel bandwidth.
Frequency Locations for Acquisition
PBCH (72 subcarriers)
Sync Signals (62 subcarriers)
Detect sync
signals
Look at center 62 subcarriers
around a target center
frequency to detect sync signals
20 MHz
f
f
PBCH reception
72 subcarriers
UE
eNB
PBCH
63

After performing power-up tests, the UE tries to obtain
timing information and frame synchronize with the
system. For this, the UE needs to obtain some timing
information indicators to indicate where the
subframe/frame begins and ends. The primary
synchronization signal is used to obtain slot
synchronization.
Radio frame structure Type 1, used for Frequency Division
Duplex (FDD) (for both full duplex and half duplex
operation), has a duration of 10 ms and consists of 20
slots with a slot duration of 0.5 ms. Two adjacent slots
form one sub-frame 1 ms long. The number of OFDM
symbols in a slot depends on subcarrier spacing and cyclic
prefix (CP) length.
Radio frame structure Type 2 is used for Time Division
Duplex (TDD) and consists of two half-frames with a
duration of 5 ms each and containing each eight slots 0.5
ms long and three special fields (DwPTS, GP and UpPTS)
that have configurable individual lengths and a total
length of 1 ms. A sub-frame consists of two adjacent slots,
except for sub-frames 1 and 6, which consist of the
DwPTS, GP and UpPTS.
The Type 1 frame structure using a normal cyclic prefix
(CP) is shown in this diagram. A 10 ms duration frame
contains 10 sub-frames (numbered 0 through 9) of 1 ms
each. A subframe is further divided into two slots of 0.5
ms each (numbered 0 through 19), thus making a total of
20 slots in each frame. Each slot contains seven OFDM
symbols (when using normal cyclic prefix). Primary
synchronization signals are transmitted on the last OFDM
symbol of time slots S0 and S10 (OFDM symbol number 6
of the first slots in Sub-frames 0 and 5).
The mechanism for using primary synchronization signal is
as follows. The mobile station searches for the eNB to
which it has the least path loss. This is accomplished by
looking for the primary synchronization signal whose
sequence in a cell can be selected from a set of three
different sequences. Since no timing information is
available, the mobile relies on matched filters to get the
best possible match. Once it matches with any primary
synchronization signal the UE has obtained slot
synchronization.
1ms
Slot Synchronization
SF
0
SF
1
SF
2
SF
5
SF
9
S0 S1 S2 S10 S19
Frame
Frame Durations = 10ms
0 1 2 ….. 5 6
0.5ms
Primary Sync Signal sequences in S0 and S10 are the same
0 1 2 ….. 5 6
0.5ms
1 of 3 Primary Synchronization sequences
64

After the primary sync signal acquisition, the UE tries to
obtain frame information and the PCI. The secondary
synchronization signal carries this information.
A secondary sync signal occupies the center 62
subcarriers of the DL channel. The Type 1 frame structure
using a normal cyclic prefix (CP) is shown in this diagram.
For the Type 1 frame structure, secondary synchronization
signals are transmitted on the second to last OFDM
symbol of slots S0 and S10 (OFDM symbol number 5 of
the first slots in sub-frames 0 and 5). The secondary sync
signal information carried on the two sub-frames within a
frame are different. This difference is used to identify the
beginning of the frame.
Frame Synchronization
Frame Frame
SF0 SF1 SF2 SF9
S0 S1 S2 S10 S19 S0 S1 S2 S 10 S19
 Secondary Synchronization Signal
T Frame = 10ms T Frame = 10ms
Tsf=1ms
Ts=0.5ms
0 1 2 ….. 5 6 0 1 2 ….. 5 6
Secondary Sync Signal sequences in S0 and S10 are different
&
SF0 SF1 SF2 SF9
65

LTE defines 504 unique Physical Layer Cell Identities.
These identities are arranged as 168 unique 62-bit
sequences representing unique cell group identities. Each
of these sequence is then scrambled with one of three 62-
bit zadoff-chu sequences to get 168 * 3 = 504 unique cell
Identities. Each eNB transmits one of the 168 unique 62-
bit sequences on the secondary sync signal and one of the
three unique scrambling sequences on the primary sync
signal (i.e., secondary sync signals are scrambled with one
of the three unique zadoff-chu sequences transmitted on
the primary sync signal). The UE first acquires the primary
sync signal to know the unique 62-bit zadoff-chu
sequence (from a set of three sequences) and get time
synchronized with the system. Using this information, the
UE tries coherent detection of the secondary sync
information to extract frame timing and the PCI. Hence,
the UE can easily decode the secondary sync signal and
identify the unique PCI.
Physical Cell ID (PCI)
Secondary Sync (NID
1)
Primary Sync (NID
2)
Physical Cell ID
0 167
0 1 2 3 4 5 501 502 503
Cell ID =3NID
1+NID
2
0 2
1 0 2
1 0 2
1
0 2
1
66

The Master information block (MIB) is important system
information that an eNB broadcasts every 40 ms with a
repeat broadcast every radio frame (10 ms) on the PBCH.
The logical and transport channel for the MIB broadcast
information are the BCCH and BCH respectively. The MIB
information is summarized below.
• Physical Layer parameters like the LTE downlink
bandwidth, the number of transmit antennas (this
information is masked with the CRC of MIB), and
PHICH configuration help the UE to read various DL
physical channels.
• The System Frame Number (SFN) helps in
synchronization and provides a source of reference to
find the system information blocks.
– For example, System Frame Number (SFNi mod
4 = 0) starts the transmission of new MIB
information on the PBCH.
– In each of these four radio frames, four OFDM
symbols x 72 subcarriers are used to send the
MIB.
– In each of these four radio frames, the same MIB
information is repeated.
– MIB content only changes at SFN mod 4 = 0.
Master Information Block (MIB)
SFN i
eNB
Example MIB Contents
• DL Bandwidth
• System Frame Number
UE
• New MIB every 40 ms TTI
• Same information
every 10ms within TTI
SFN i+1 SFN i+2 SFN i+3
PBCH
4 OFDM symbols x 72 subcarriers
67

Exercise: Locate the Signals/Channel
• Map the Primary and Secondary sync signals on
the LTE resource grid.
• Map the Physical Broadcast Channel on the LTE
resource grid.
• Questions to ask yourself include:
– Which and how many RBs?
– Which symbols?
68

Acquiring SIBs
69

Once the UE has read the PBCH, it needs to read the
PCFICH. The PCFICH carries the Control Format Indicator
(CFI) field, which indicates to the UE the number of OFDM
symbols in the sub-frame that will carry the PDCCH. The
PDCCH carries information about the radio resources for
the PDSCH that will carry the SIB information. The CFI
information is a 2-bit number, containing a number value
of one to four. When using FDD, LTE supports a maximum
of three OFDM symbols to carry the PDCCH when the total
number of PRBs in the channel is 10 or more.
Why does a UE Need to look for the PCFICH here?
• Where are SIBs? PDSCH
• How to find PDSCH? PDCCHs
• How many symbols occupied by PDCCHs? PCFICH
Control Format Indicator (CFI)
eNB
Answers the Q:
How many OFDMA symbols are
occupied by PDCCHs?
UE
70

Resource Element Groups (REGs) are used for defining
the mapping of control channels to resource elements.
Frequency diversity is obtained for the control channel by
distributing each control channel element over the entire
bandwidth of all control symbols, basically for reliability
reception.
A REG is a contiguous grouping of four resource elements
not counting resource elements used for reference
signals. Assuming two antenna ports in a cell, there are
two resource element groups in the first OFDM symbol of
a physical resource block. In the other OFDM symbols
there are three resource element groups per physical
resource block. If there are four antennas in a cell, then
the number of REGs in the second OFDM symbol would
only be two per PRB.
Why? Control channel mapping and frequency diversity.
Resource Element Groups
11 4 4 4 4
10 3 3 3 3
9 2 2
8 2 2 1 1
7 1 1 4 4
6 3 3
5 4 4 2 2
4 3 3 1 1
3 4 4
2 2 2 3 3
1 1 1 2 2
0 1 1
OFDM symbols
0 1 2 3
Sub
carriers
• 1 REG: 4 modulation
symbols
: Reference Signals
REG
71

Based on the information transmitted on the PCFICH, the
UE knows how to read the PDCCH channel information.
The DL Control Information (DCI) records in the PDCCH
carry the scheduling information about resources
allocated on the PDSCH for carrying SIBs, user signaling
information and user traffic. The CRC of a DCI indicating
PDSCH resources used for SIBS is scrambled with a 16-bit
System Information-Radio Network Temporary Identity (SI-
RNTI) (0xFFFF).
Several DCI formats have been defined to carry DL control
information for different transmission modes on the
PDSCH.
DL Control Information (DCI)
eNB
• Conveys PDSCH resource allocation for SIBs
(and user signaling/traffic)
• SI-RNTI: Used to scramble CRC of the DCI to
identify the DCI describing SIB resources
UE
72

SIB type 1 is important system information that an eNB
broadcasts every 80 ms with a repeat broadcast every two
radio frames (20 ms) on the PDSCH. SIB type 1 messages
help the UE in cell selection. The logical and transport
channels for the SIB that broadcast information are the
BCCH and DL-SCH respectively. The parameters in SIB1
include:
• PLMN IDs of the network operator,
• Cell ID and tracking area code information useful for
the UE on mobility,
• Cell barring status to indicate whether the UE can
latch on to the cell or not,
• Q-Rxlevmin, a scalar quantity that helps cell selection,
and
• Scheduling information that indicates when other
SIBs transmission starts and what the periodicity is.
System Information Block Type 1
eNB
UE
• 80ms TTI
• Same info repeated
every 20ms
SFN i SFN i+1 SFN i+2 SFN i+3 SFN i+4 SFN i+5 SFN i+6 SFN i+7
• PLMN ID
• Cell ID, Tracking area code
• Cell Barring status
• q-Rxlevmin
• Scheduling info for other SIBS
• PLMN ID
• Cell ID, tracking area code
• Cell barring status
• q-Rxlevmin
• Scheduling information for
other SIBs
73

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